Electro-optic effects with sub-microsecond switching speeds can be achieved using the technology of ferroelectric liquid crystals (FLCs) of N. A. Clark and S. T. Lagerwall (1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures prepared using FLC materials having not only high speed response (about 1,000 times faster than currently used twisted nematic devices), but which also exhibit bistable, threshold sensitive switching. Such properties make FLC based devices excellent candidates for light modulation devices including matrix addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays.
Smectic C liquid crystal phases composed of chiral, nonracemic molecules possess a spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Myer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions. In ferroelectric liquid crystal display devices, like those of Clark and Lagerwall, appropriate application of an external electric field results in alignment of the chiral molecules in the ferroelectric liquid crystal phase with the applied field. When the sign of the applied field is reversed, realignment or switching of the FLC molecules occurs. This switching can be employed for light modulation. Within a large range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (P), and directly proportional to orientational viscosity. High switching speeds are then associated with FLC phases which possess high polarization density and low orientational viscosity.
Optics may provide an alternative to electronics in data communication and processing applications where high data rates are achieved through either high bandwidth channels or highly parallel processing. To this end a variety of devices for the generation and detection of intensity modulated light have been developed. However, the development of computer systems based on optics technology has been hindered by deficiencies in two key components: the optical crossbar switch and the spatial light modulator (SLM). Both of these components require a large number of light modulating elements, ideally rapidly switchable, in a small area. Envisioned systems could use crossbar switches with up to N=1000 lines, requiring between about 10.sup.4 [Nln(N)] to 10.sup.6 [N.sup.2 ] switching elements, or SLMs with up to N.sup.2 =10.sup.6 elements. Any switching element considered for these components must meet certain requirements. The individual switching elements must be small in size so that the complete device will not be unwieldily large. The elements must be cable of switching quickly so that the device can be reconfigured in a reasonable time. The elements should afford high contrast between the "on" and "off" states (&gt;1000:1 required for a 1000 element crossbar switch). The energy required to switch the element must be small enough that the heat generated by reconfiguration of the whole device, within the desired time, does not generate more heat than can be feasibly dissipated. The elements must share control lines or in some way be addressable in parallel so that the number of control lines remains small compared to the number of signal lines. The elements must be manufacturable by techniques that produce all the elements needed for a complete device in parallel so that the cost of the device in not prohibitive.
Conventional electro-optic materials (e.g., LiNbO.sub.3) can switch in a few picoseconds but are only weakly birefringent and thus require long optical interaction lengths or high drive voltage which produces large switching energy to achieve required high contrast. Most other optical switching materials suffer from similar shortcomings.
Most effort in the development of FLC materials has been directed towards flat panel display applications. The optimal characteristics for FLC materials used in such displays include high spontaneous polarization and low orientation viscosity which are required to achieve fast switching, tilt angles of 22.5 which result in maximum contrast between crossed polarizers, low birefringence which facilitates construction of a desirable thickness panel and broad temperature range of the desired FLC phase.
Unfortunately, several of these parameters are far from optimal for FLC materials useful in waveguides, integrated optics, and spatial light modulators. High polarization and low viscosity are desired for both display and optical switching FLC applications. However, enhanced performance in optical switching applications is correlated with high total refractive index change between the switched states which is associated with high birefringence and large tilt angles.
A particular type of FLC display device, a dichroic display device containing color switching elements incorporating mixtures of FLCs with dichroic dyes, also requires high tilt FLC material (tilt angle of 45.degree.) to achieve highest contrast (see Ozaki et al. (1985) Jpn. J. Appl. Phys. Part I 24 (Suppl. 24-3):63-65).
A basic requirement for application of ferroelectric liquid crystals in any device is the availability of chemically stable liquid crystal materials which exhibit ferroelectric phases over a substantial temperature range about room temperature. In some cases, the ferroelectric liquid crystal compound itself will possess an enantiotropic or monotropic ferroelectric (chiral smectic C*) liquid crystal phase. Ferroelectric liquid crystal mixtures possessing chiral smectic C* phases with useful temperature ranges can also be obtained by admixture of chiral, nonracemic compounds, designated ferroelectric liquid crystal dopants, into a liquid crystal host material (which may or may not be composed of chiral molecules). Addition of the dopant can affect the ferroelectric polarization density and/or the viscosity of the C* phase and thereby affect the switching speed. To achieve fast switching, desirable FLC dopants are molecules which impart high ferroelectric polarization density to an FLC material without significantly increasing the orientational viscosity of the mixture. For applications requiring high tilt angle materials it is desirable to have FLC materials which combine the properties of high tilt angle with fast switching speed and broad room temperature smectic C* phases.
Thermotropic liquid crystal molecules typically possess structures which combine a rigid core coupled with two relatively "floppy" tails (see Demus et al. (1974) Flussige Kristalle In Tabellen, VEB Deutscher Verlag fur Grundstoffindustrie, Lebzig for a compilation of the molecular structures of LC molecules). FLC materials have been prepared by the introduction of a stereocenter into one of the tails, thus introducing chirality. The first FLC compound to be characterized was DOBAMBC (Meyer et al., supra) which contains an (S)-2-methylbutyloxy chiral tail. Pure DOBAMBC exhibits a smectic C* phase with a ferroelectric polarization of -3 nC/cm.sup.2.
The structures and polarization of several known smectic C* materials, including several containing phenylbenzoate cores, have been summarized in Walba et al. (1986a) J. Amer. Chem. Soc. 108:5210-5221, which also discusses a number of empirical correlations between molecular structure and FLC properties.
There are several reports of compounds containing phenylbenzoate and related cores coupled to chiral tail units which possess monotropic smectic C* phases displaying fast switching speeds at room temperature, or which can be employed as FLC dopants to induce high polarization and fast switching speeds when combined in mixtures with FLC host materials. Walba et al., U.S. Pat. No. 4,556,727 reports phenylbenzoates having non-racemic 2-alkoxy-1-propoxy tails. Eidman and Walba, U.S. Pat. No. 4,777,280 reports chiral 1-cyanoalkoxy phenylbenzoates. Walba and Razavi, U.S. Patent 4,695,650 reports chirally asymmetric reverse ester phenylbenzoates having chiral 1-haloalkyl tail units. Wand and Walba, U.S. patent application Ser. No. 164,233, filed Mar. 4, 1988, now allowed reports chirally asymmetric FLC materials having 2-haloalkoxy, 2,3-dihaloalkoxy or 2,3,4-trihaloalkoxy tails incorporated into a suitable core such as those based on biphenyl, phenylbenzoate, biphenylbenzoate or phenylpyrimidine moieties.
Walba and Razavi, U.S. Pat. No. 4,835,295 filed Sept. 21, 1987, discloses chirally asymmetric phenyl- and biphenylbenzoates having chiral 2,3-epoxy alkyl or 1-halo-2,3-epoxy alkyl tails which are useful for the preparation of FLC materials which possess smectic C* phases and have high polarization density. Tilt angles of the smectic C* phases of these FLC mat.RTM.rials have not been reported.
Walba et al. (1986) J. Amer. Chem. Soc. 108:7424-7425 and Walba and Vohra, U.S. Pat. Nos. 4,648,073 and 4,705,874 disclose ferroelectric smectic liquid crystal compounds possessing a high ferroelectric polarization density having chiral tail units derived from (2,3) alkyloxiranemethanols and achiral phenylbenzoate and biphenyl core units. The ferroelectric crystal compounds reported have the following general formulas: ##STR2## where R is an alkyl of one to seven carbon atoms and R' is an alkyl of five to twelve carbon atoms and Ar is phenylbenzoate or biphenyl. Tilt angles of the smectic C* phases of these FLC materials have not been reported.
Hemmerling et al. (1988) European Patent Application, Pub. No. 263437 refers to chiral aryl-2,3-epoxyalkylether FLC compounds having phenylpyrimidine or phenylpyridazine cores of the formula: ##STR3## where A is a diazine-2,5,-diyl or diazine-3,6-diyl, R.sup.4 includes straight chain or branched alkyl groups and R.sup.1 includes straight chain or branched alkyl groups having one to twelve carbons. The properties, including tilt angle, of several FLC mixtures containing certain of these compounds are provided. All of the FLC mixtures described have tilt angle less than 30.degree..
Ichihashi et al. (1988) European Patent Application, Pub. No. 269062 describes FLC compositions reported to have tilt angles between 30 to 60. and superior alignment properties. The authors state that most known FLC compounds (single component materials) having a high tilt angle , i.e. greater than or equal to 30.degree. in the smectic C* phase, do not posses a higher temperature smectic A phase, while those having such a smectic A phase have low tilt angle, i.e. less than 30.degree.. It is, thus, inferred that tilt angle depends on the ordering of phases, in particular on the type of phase occurring above the C* phase. It is reported that high tilt smectic C* compositions which posses a higher temperature smectic A phase can be obtained by admixture of a chiral or achiral smectic C compound also having a smectic A phase having a temperature range within 40.degree. C. with a chiral smectic C compound having no smectic A phase. The presence of a smectic A phase is described as useful for alignment of an FLC material. The authors also report that tilt angle of a composition is an approximately additive function of the tilt angles of the individual components. Although the tilt angles of a number of smectic C compounds including several having phenylbenzoate and related cores are reported, no correlation between tilt angle and chemical structure is noted.
In a related reference, Furukawa et al. (1988) European Patent Application Pub. No. 220747 refers to a method for controlling the tilt angle in ferroelectric smectic C mixtures. The reference contains a list of FLC compounds, including a number of compounds having phenylbenzoate and related cores, giving phase diagrams and in many cases tilt angles. This reference also refers to the correlation between the presence of smectic A phases with tilt angles less than 30.degree.. A method for controlling the tilt angle of an FLC mixture by adjusting the composition of the mixture such that a smectic A phase is present (for low tilt angle mixtures) or absent (for high tilt angle mixtures).